This invention relates to a catalyst component for polymerization of alpha-olefins and a method for using the same. More specifically, this invention relates to a carrier-supported titanium catalyst component which exhibits high activity and high stereospecificity and which can produce polymers having a uniform particle diameter and to a process for using the same.
In the production of olefin polymers, especially polypropylene, using Ziegler-Natta type catalysts, many attempts have been made heretofore to obtain catalysts exhibiting high activity and high stereospecificity for the ultimate purpose of offering the resulting polymers as final products without any further treatment.
In recent years, supported catalysts comprising titanium supported on various carriers, particularly magnesium compounds have been developed. These supported catalysts greatly increase the polymerization ability of the titanium component as compared with conventional Ziegler-Natta type catalysts. However, no ultimate catalyst has yet been perfected.
Attempts have been made to obtain catalysts capable of controlling the particle size of the resulting polymer while retaining their high activity and high stereospecificity.
It is said that the shape of a polymer obtained by using a Ziegler-Natta type catalyst is generally affected by the shape of the catalyst component used. With respect to supported catalysts there are few methods available for controlling shape.
As far as supported catalysts having a magnesium compound, particularly magnesium chloride, as a carrier are concerned, there has been known, for example, a method which comprises spray-drying molten magnesium chloride to form spherical particles, and supporting titanium tetrachloride thereon in the suspended state [(1) Japanese Laid-Open Patent Publication No. 65999/74, (2) Japanese Laid-Open Patent Publication No. 38590/77], and a method which comprises supporting titanium tetrachloride on classified powdery magnesium chloride in the suspended state [(3) Japanese Laid-Open Patent Publication No. 127185/76].
The catalysts obtained by the methods (1) and (2) relate mainly to the case of producing polyethylene. These catalysts exhibit low stereospecificity when used for producing polypropylene. Method (3) relates mainly to a catalyst for the polymerization of ethylene. The particle characteristics of the resulting polymer are good, but the polymerization efficiency of the catalyst is not necessarily high.
The specification of Japanese Laid-Open Patent Publication No. 146292/78 describes a method which comprises reacting metallic magnesium and an organic halide or an organic compound of magnesium with an ortho-silicic acid ester to form a magnesium compound having controlled particle characteristics, treating this compound with an electron donor compound and/or a halogen compound to obtain a substance containing a magnesium halide, especially magnesium chloride, as a main ingredient, and supporting titanium tetrachloride on this substance. Although the particle size distribution of the resulting polymer is considerably improved, the activity and stereospecificity of the catalyst are never high and the method cannot be said to be economically advantageous because the ortho-silicic acid ester, which is essential in this method, is a special compound and is difficult to obtain.